Project 3: Continuous Wave Electron Paramagnetic Resonance Imaging. Summary of Progress: Existing CW methodologies involve using a constant vector field gradient and relatively slow sweep of magnetic field and the use of field modulation, a signal detection method known as phase-sensitive detection. The use phase sensitive detection, which mandates scans which are slow (seconds) making the image data acquisition times for multidimensional imaging unacceptably long (more than 30 minutes) for in vivo applications.. We have developed a novel CW imaging strategy at 300 MHz frequency that incorporates three approaches to collect image data with increased spatial, temporal, and spectral resolution and thereby improve the sensitivity of measurement for unit time. Firstly, the spectral data acquisition is carried out using direct-detection strategies by mixing the signals to base-band and directly acquiring the data with a fast-digitizer. Secondly, the projections are acquired using fast sinusoidal magnetic field sweep under gradient magnetic fields. Thirdly, gradients themselves are circularly polarized and applied in synchrony with the sinusoidal sweep. With such an approach the image data collection becomes extremely efficient. The sensitivity improvements is further accentuated by implementing digital signal processing (DSP) techniques, which eliminate several sources of noise originating from analog devices in the spectrometer. The modular design of the CW spectrometer consists of optimized DSP (digital signal processing) transmit and receive systems, that allows a choice of pre-programmed 2D and 3D and spectral-spatial image data collection at a number of frequencies and desired number of projections to be selected from the transmitter module, an interactive DSP receive and image reconstruction system that allows optimizing the extent of signal averaging for satisfactory image quality, etc. The applicability of such a fast imaging strategy in CW EPR that almost reaches the speed of time-domain (pulsed EPR) modality has already been tested successfully in phantom and in vivo imaging experiments. Currently we use 1.2 kHz sinusoidal field sweeps of 1.5 mT amplitude, and 4.8 kHz rotating gradients with maximum amplitude 40mT/m. With this configuration which is constrained by our existing gradient amplifiers, the measurement times for EPR images are as follows: Single 64 point projection: 205 s; 2D image 64x64 52.4 ms; 3D image 63x64x64 3.35 s. These speeds approach that of time domain modalities. However, since CW spectroscopy is not handicapped by the line width (or transverse relaxation times as in the pulsed mode) this rapid-scan rotating-gradient direct detection strategy will be very powerful in fast measurement of nitroxides, spin trapped nitric oxide, and other biologically important free radicals that cannot be easily detected by pulsed methods at low frequencies. We are further exploring the possibility of increasing the sweep and gradient rotation frequencies by procuring higher power AC amplifiers. In the meanwhile applications of this technique for following tissue redox status, CW EPR oxymetry and studies on trapping of nitric oxide, etc, are on the anvil. Attempts are being made in selecting slices from within the object to restrict the measurement to limited locations and to further improve temporal resolution. It is to be pointed that this resonator assembly is also amenable for MRI-EPRI co-registration.